Communication signals are typically classified according to modulation type. Each of the various forms of modulation has its own set of advantages and disadvantages relative to a specific application for which it will be used. Some factors to consider in choosing a particular form of modulation include bandwidth, power consumption requirements, and the potential for signal propagation errors and recovery of the original information. For digital data, whether a separate clock signal is required or the modulated signal is self-clocking may be important. The relative simplicity or complexity of the modulating and demodulating equipment or circuitry may also be a factor in the decision. Low power consumption is particularly sought for use with capacitive-loaded transmission lines.
Each type of modulation has specialized encoder circuitry for performing the modulation. For example, U.S. Pat. No. 6,439,679 to Roylance discloses a pulse width modulator (PWM) circuit that includes a clock delay circuit with multiple taps, a tap selection circuit making a selection based on a pulse code input, and a transition generating circuit that generates the PWM output from the selected delayed clock. The pulse code input is interpreted by a timing instruction processing instruction circuit that generates a corresponding vector output that indicates when timing transitions associated with the PWM output should occur.
U.S. Pat. No. 5,442,664 to Rust et al. describes a modulator used for RF interference reduction that produces clock pulses with a series of different phase displacements by using a delay chain with a plurality of taps to provide various phase delays of a clock, an up/down counter serving as a selector module that sequentially renders active different ones of its output lines, and a multiplexer circuit made up of AND gates with one input receiving the various delayed clocks and another input receiving the selector output lines and OR gates propagating the selected delayed clock to the output.